1. Field of the Invention
The present invention relates to an injection timing control apparatus for distributor type fuel injection pumps, particularly to an injection timing control apparatus for distributor type fuel injection pump which uses a servo valve.
2. Description of the Prior Art
With conventional fuel injection pumps, an increase in engine speed has to be matched by an advance in the fuel injection timing to ensure that the fuel is injected with the optimum timing. For this, the fuel injection pump is equipped with a fuel injection timing control device (timer).
Usually, a pressure control valve is used to produce a rise in the pressure in the pump chamber corresponding to the increase in the engine speed, and the injection timing is controlled by controlling the timer, using this pump chamber pressure as the pressure source.
There are timers which are provided with servo valve arrangements. In the injection timing adjustment apparatus for a distributor type fuel injection pump according to Japanese Patent Laid-open Application 58-32928/1983, for example, to prevent the torque reaction from the plunger cam disk setting up vibration in the roller, a servo valve is used to temporarily break the connection between the pump chamber and the timer high-pressure chamber, creating a hydraulic lock.
A conventional injection timing adjustment apparatus for a distributor type fuel injection pump will now be briefly described, with reference to FIG. 7. FIG. 7 is a longitudinal cross-sectional view of a conventional injection timing adjustment apparatus 1 for a distributor type fuel injection pump. A roller holder 4 is arranged in a pump chamber 3 formed in a pump housing 2. A roller 5 is rotatably held by the roller holder 4, and abuts the cam disk of a plunger (not shown) which is mounted in the pump housing 2. A drive shaft (not shown) drives the plunger in a reciprocating rotational motion to intake and inject the fuel.
A lever 6 is affixed to the roller holder 4. The contact between the roller 5 and the cam disk can be altered by adjusting the angle of the lever 6 to change the range of the operating cam surface and thereby control the fuel injection timing.
The end of the lever 6 contacts a timer piston 8 via a slide roller 7. This timer piston 8 is capable of reciprocating motion in a timer cylinder 9 formed in the pump housing 2. One end of the timer cylinder 9 is sealed shut by a lid member 10 and an O-ring 11, forming a timer high-pressure chamber 12 between the timer piston 8 and the lid member 10. The other end of the timer cylinder 9 is also sealed by a lid member 13 and a O-ring 14, forming a timer low-pressure chamber. The timer low-pressure chamber 15 is connected to a low-pressure section at the intake side of the fuel feed pump (not shown).
A timer piston play damper spring 16 is provided between the lid member 13 and the timer piston 8 which together with a servo valve control spring 19, described below, urges the timer piston 8 towards the timer high-pressure chamber 12.
A guide bushing 17 is affixed in the timer piston 8. The guide bushing 17 contains a servo valve 18 which can be moved to the left and right, with respect to the drawing. A servo valve control spring 19 is provided between the servo valve 18 and the lid member 13.
Formed in the timer piston 8 is a high-pressure hydraulic passage 20 which communicates to the timer high-pressure chamber 12. In the guide bushing 17 are formed an inflow high-pressure aperture 21 that communicates with the pump chamber 3, and a communicating aperture 22 that communicates with the high-pressure hydraulic passage 20. Formed in the servo valve 18 is a low-pressure-side communicating passage 23 which can be connected to the communicating aperture 22 and to the timer low-pressure chamber 15.
In FIG. 7 the injection timing adjustment apparatus 1 for a distributor type fuel injection pump thus configured is shown in the maximum retarded state, in which the rise in pressure in the pump chamber 3 is exerted on the servo valve 18 via the high-pressure aperture 21, moving the servo valve 18 to the right against the force of the servo valve control spring 19 and bringing the high-pressure aperture 21 into communication with the communicating aperture 22. As a result, the high pressure in the pump chamber 3 extends, via the high-pressure hydraulic passage 20, to the timer high-pressure chamber 12, also moving the timer piston 8 to the right. This causes the lever 6 to rotate counterclockwise, which changes the range of contact between the roller 5 and the face of the cam disk, enabling the timing to be advanced. This position is stabilized when the movement of the timer piston 8 to the right breaks the communication between the high-pressure aperture 21 and the communicating aperture 22.
A pressure drop in the pump chamber 3 will cause the servo valve 18 to be moved to the left by the force of the servo valve control spring 19. This brings the communicating aperture 22 into communication with the high-pressure hydraulic passage 20, allowing fluid under high pressure in the timer high-pressure chamber to flow into the timer low-pressure chamber 15. This moves the timer piston 8 to the left and rotates the lever 6 clockwise, retarding the timing. This conventional servo valve timer, that is, the injection timing adjustment apparatus 1 for a distributor type fuel injection pump, features good response compared with a standard timer that does not have a servo valve 18, and other advantage include low hysteresis and the small effect that advancing the timing has on changes in the fuel injection amount. On the other hand, as the servo valve 18 is located on the timer low-pressure chamber 15 side of the lever 6, assuming that the direction of rotation of the engine, i.e. of the cam disk and the plunger is the same, a servo valve 18 cannot be provided in cases where the engine is located on the timer low-pressure chamber 15 side as the engine cylinder block would be in the way. Thus, the servo valve arrangement is only applicable to a limited number of engine types.
This problem is not limited to the above-described injection timing control apparatus 1 in which the servo valve 18 is driven by the hydraulic pressure in the pump chamber 3, but also arises in the case of other injection timing control apparatus that use solenoids and the like. This is explained in the following, in which parts which are the same as those in FIG. 7 have the same reference numerals.
FIG. 8 shows an injection timing adjustment apparatus 30 in which a servo valve 33 is screwed into a rod 32 of an actuator such as linear solenoid 31 and is provided with an actuator play damper spring 34. In addition, a timer piston play damper spring 35 is provided between the timer piston 8 and the linear solenoid 31.
A high-pressure hydraulic passage 36 is formed in the timer piston 8, a high-pressure hydraulic passage 38 in the guide bushing 37 and an annular groove 39 in the servo valve 33. In addition, provided in the servo valve 33 are a center low-pressure hydraulic passage 40 which communicates with the annular groove 39, and a radial low-pressure hydraulic passage 41 which communicates with the center low-pressure hydraulic passage 40.
In this arrangement, when the servo valve 33 is moved to the right by the linear solenoid 31, the pump chamber 3 is brought into communication with the timer high-pressure chamber 12 via the high-pressure hydraulic passage 36, the high-pressure hydraulic passage 38, the annular groove 39, the communicating aperture 22 and the high-pressure hydraulic passage 20, whereby the timer piston 8 is moved to the right by the pump chamber pressure, advancing the timing.
Conversely, when the servo valve 33 is moved to the left by the linear solenoid 31, the timer high-pressure chamber 12 is brought into communication with the timer low-pressure chamber 15 via the high-pressure hydraulic passage 20, the center low-pressure hydraulic passage 40 and the radial low-pressure hydraulic passage 41, moving the timer piston 8 to the left and retarding the timing.
The advantage of this type of injection timing adjustment apparatus 30 include structural simplicity, good precision, and good resistance to interference from the drive reaction (torque reaction). However, as in the case of the injection timing adjustment apparatus 1 described above, the servo valve 33 and the linear solenoid 31 are located on the timer low-pressure chamber 15 side of the lever 6, with the linear solenoid 31 protruding quite considerably, making it impossible to avoid interference with the engine.